Variable ventilation ages in the equatorial Indian Ocean thermocline during the LGM

Variations of atmospheric CO2 during the Pleistocene ice-ages have been associated with changes in the drawdown of carbon into the deep-sea. Modelling studies suggest that about one third of the glacial carbon drawdown may not be associated to the deep ocean, but to the thermocline or intermediate ocean. However, the carbon storage capacity of thermocline waters is still poorly constrained. Here we present paired 230Th/U and 14C measurements on scleractinian cold-water corals retrieved from ~ 450 m water depth off the Maldives in the Indian Ocean. Based on these measurements we calculate ∆14C, ∆∆14C and Benthic-Atmosphere (Batm) ages in order to understand the ventilation dynamics of the equatorial Indian Ocean thermocline during the Last Glacial Maximum (LGM). Our results demonstrate a radiocarbon depleted thermocline as low as -250 to -345‰ (∆∆14C), corresponding to ~ 500–2100 years (Batm) old waters at the LGM compared to ~ 380 years today. More broadly, we show that thermocline ventilation ages are one order of magnitude more variable than previously thought. Such a radiocarbon depleted thermocline can at least partly be explained by variable abyssal upwelling of deep-water masses with elevated respired carbon concentrations. Our results therefore have implications for radiocarbon-only based age models and imply that upper thermocline waters as shallow as 400 m depth can also contribute to some of the glacial carbon drawdown.


Scientific Reports
| (2023) 13:11355 | https://doi.org/10.1038/s41598-023-38388-z www.nature.com/scientificreports/ of uranium, allowing accurate age determination through 230 Th/U dating 26 . Combined 230 Th/U and 14 C measurements enable us to determine past ocean 14 C/ 12 C ratios and thus are a proxy for accurate and precise ∆ 14 C and corresponding ventilation ages 29,30 . Moreover, CWC aggradations often occur near the boundaries of thermocline and intermediate waters, which are sensitive to large-scale oceanographic perturbations 31 .
Here we present the first ventilation ages derived from paired 230 Th/U and 14 C measurement on CWCs retrieved from thermocline waters (here: the permanent thermocline is the transitional zone where surface and deep waters mix) at 450 m water depth off the Maldives in the equatorial Indian Ocean 32 (Fig. 1). These measurements allow for the determination of ∆ 14 C and Benthic-Atmospheric (B atm ) 14 C ventilation ages for each sample (based on IntCal20 33 ). Finally, we compare our results with glacial radiocarbon simulations applying an ocean general circulation model including ∆ 14 C 32 . The Indian Ocean is only ventilated from the south and thus a cul de sac, making it an important compartment for thermohaline circulation and especially for reconstructing past ∆ 14 C of thermocline waters originating in the Southern Oceans (Fig. 2). Our new coral data provides answers to questions regarding how variable equatorial Indian Ocean thermocline ventilations ages have been and whether they have contributed to the drawdown of carbon during the LGM.  . Also shown is on the same section the salinity distribution. Data was taken from the global dataset GLODAP v2. 65 (Table 1s).The corresponding radiocarbon ages (calibrated against IntCal20 33 ) are systematically older and reveal ages from 22.071 ka to 23.503 ka (Fig. 2, Table 2s). Our dataset reveals two prominent features. Firstly, Indian Ocean thermocline waters off the Maldives in 450 m water depth appear to be extremely variable with a range in the calculated ∆ 14 C between + 109‰ and + 392‰ within a time span of less than 1.5 ka. Secondly, they are depleted compared to the IntCal20 33 atmospheric 14 C curve, and most of the (surface) Marine20 34 curve at their corresponding calendar ages (Fig. 3). This 14 C depletion of thermocline water is shown in ∆∆ 14 C (i.e. the ∆ 14 C difference between atmosphere and corals) values as low as − 250‰ to − 345‰ , corresponding to B atm ages of up to 2100 years. The observed variability in B atm ages cannot be related to species, but tend to cluster with higher B atm ages at the slightly deeper site (Malé Vaadhoo channel), although both are only < 100 km apart from each other and are both located on the eastern side of the Maldives. Moreover, our ∆∆ 14

Comparison with model results
For further analysis, we simulated the temporal evolution of radiocarbon in the equatorial thermocline. Our coral based ∆∆ 14 C (supplementary material) values and B atm ages broadly agree with the radiocarbon simulations but some discrepancies are visible ( Fig. 5 and 2s). In particular, the temporal variability of simulated B atm ages is considerably smaller than reconstructed. Simulated B atm ages vary roughly from 1000 to 1500 years in the interval covered by the corals, whereas the corals exhibit a B atm range from 500 to 2300 years. Correspondingly, within an interval of less than 1.5 ka, the coral based B atm ages are outside the uncertainty bounds spanned by the various model scenarios (Fig. 5, ∆∆ 14 C in the supplementary material).
This indicates that the simulations underestimate the past radiocarbon variability of the Indian Ocean thermocline. While previous 14 C simulations for the LGM were roughly consistent with benthic 14 C values reconstructed on other locations 41 , our new data from the Indian Ocean highlights that the 14 C history of glacial thermocline waters is complex. Thermocline waters are at the transient zone between the surface mixed layer and the deepocean. Especially in the glacial ocean, where deeper waters stored additional radiocarbon depleted carbon 8,9,11 , the equatorial thermocline of the Indian Ocean tends to reflects both atmospheric ∆ 14 C and deep ocean ∆ 14 C. However, even if radiocarbon depleted but carbon rich deep-water reservoirs are a pervasive feature of the glacial ocean, high ventilation ages in near surface waters are a difficult phenomenon to explain. In the following, we consider three hypotheses that could explain the variability and depletion of 14 C reconstructed for the glacial thermocline of the Indian Ocean: (1) in-situ aging, (2) advection of 14 C-depleted mid-depth water masses, as well as (3) 35 , and southwest Australia 23 and from the Maldives (this study, from the shallowest water depth of 450 m). For comparison, the ∆ 14 C record are plotted against the calibration curves IntCal20 33 (black) and Marine20 34 (grey), (B) Calculated ∆∆ 14 C (∆ 14 C against atmospheric ∆ 14 C at the respective interval) plotted together with the above-mentioned records. Note we only plot ∆ 14 C and ∆∆ 14 C reconstructions that are based on paired 230 Th/U and 14 C analyses. Note we have taken out one data point at ~ 17.5 by ref 23

In situ aging
Lowest ventilation ages recorded in our dataset plot near or in-between the Intcal20 33 and Marine20 34 ∆ 14 C curves as expected for thermocline water masses that have been in contact with the atmosphere. However, the observed variability in ventilations ages suggest a strong but variable aging of thermocline waters. Can the observed radiocarbon decline be explained by an in-situ aging from an isolated thermocline water? We discount this hypothesis for the following reasons, (a) our sites here are not horizontally isolated from other ocean basins, (b) in-situ aging is at odds with the amplitude and rapidity of our reconstructed ∆∆ 14 C variations (about 300‰ within 1500 years).

Advection of intermediate and mode waters
The decadal to centennial scale variability seen in our 14 C record could be explained by the advection of southern sourced mode waters 16,17 . Here, the principal mechanism is the upwelling of carbon and nutrient-rich water in the Southern Ocean, which is subsequently transported to the equatorial thermocline by the Antarctic Intermediate Water (AAIW) and the Subantarctic Mode Water (SAMW) 16,17 . In the Equatorial Pacific, the advection of such Southern Ocean radiocarbon depleted waters was synchronous with deep-water ventilation changes 22 . However, even though this mechanism has been proposed for periods of abrupt climatic perturbations such as the Younger Dryas and Heinrich Stadials I and II, reconstructed ventilation ages of the intermediate northern Indian Ocean do not exhibit any larger excursions during the LGM 17 . Further evidence comes from a neodymium isotope based reconstructions showing, that advances of AAIW in the equatorial Indian Ocean are restricted to the deglaciation and did not occur during the LGM 42 .
It has been suggested that increased glacial reservoir ages could be related to decreased air-sea equilibration during the LGM 43 . However, the amplitude of our reconstructed ventilation changes rather supports the  www.nature.com/scientificreports/ hypothesis of altered glacial deep-sea overturning and increased CO 2 storage, as recently suggested by a comprehensive compilation of glacial deep-sea 14 C records 11 . Nevertheless, with the present dataset we cannot rule out that radiocarbon depleted mid-depth waters, either SAMW or AAIW, may have partly contributed to the observed variability in the thermocline ventilations ages.

Abyssal upward mixing of 14 C depleted carbon
Our reconstructed variable and increased ventilation ages of thermocline waters in the equatorial Indian Ocean during the LGM can be attributed to upward mixing of deep waters. As the Indian Ocean is solely ventilated from the south 44 , modern Indian Deep Water (IDW) is formed from abyssal waters such as Antarctic Bottom Water via diapycnal mixing in the interior 37-45. , thereby increasing the volume of southern sourced water masses at shallower water depths (Figs. 2 and 6). Thus, abyssal upwelling controls the distribution pattern of DIC and 14 C concentrations, revealing a gradual aging that ends up in the upper deep-water of the northern Indian Ocean [45][46][47][48][49] . Consequently, modern IDW is considered as a key supplier of carbon for the Southern Ocean upwelling 37 . During the last glacial period and in particular during the LGM, southern sourced waters expanded into deep and abyssal depth of the Indian Ocean, displaced the ambient Atlantic source water mass and thereby significantly increased the carbon storage capacity of the deep 39,[50][51][52] . A replacement by a southern sourced deep-water mass could therefore be accompanied by poor ventilation and in turn by a lack of oxygen replenishment. As a water mass remains isolated from the atmosphere, 14 C decays while oxygen is consumed due to oxidation of organic matter. Thus, we would expect water mass aging to be accompanied by decreasing oxygen concentrations. Indeed, there is evidence for anoxic bottom waters during the LGM in the deep Indian Ocean 49,52 . Accordingly, poorly ventilated deep-water masses and anoxic conditions point towards an extremely radiocarbon depleted deep-water in the abyssal and deep Indian Ocean, which may have extended into the thermocline leading to temporally very variable ventilation ages.
During the LGM, radiocarbon depleted but carbon rich waters have been identified in the Southern Ocean such as the Drake Passage, in the Indian Sector of the Southern Ocean as well as off Tasmania, but with B atm ages lower than < 3000 years 10,35,36 . Mid-depth waters tend to shoal on the pathway into the tropics 44 . Thus, a 14 C Southern Ocean signal would be diluted with 14 C enriched low-latitude surface waters during the pathway into www.nature.com/scientificreports/ the Indian Ocean, making it difficult to generate B atm ages of up to ~ 2100 years in the equatorial thermocline. This would in turn imply that a substantially older deep-water mass is required to explain the observed 14 C depletion in thermocline waters. Very high ventilation ages near the LGM (> 4000 years) have been identified in the (SW) Pacific Ocean 8,14,39 , but also in the northern deep and abyssal Indian Ocean by using fossil foraminiferal ∆ 14 C ages 51,53 . These extremely old deep-and abyssal water masses may thus be the most likely potential radiocarbon depleted source to cause, by upward mixing with the overlying water mass, the accumulation of 14 C-depleted DIC in the equatorial thermocline of the Indian Ocean (Fig. 6). Moreover, these radiocarbon depleted thermocline waters at the LGM may have also contributed to the deglacial release (Younger Dryas and Heinrich Stadial 1) of 14 C depleted intermediate water masses in the Arabian Sea 17 , implying strong local differences of carbonate system characteristics.
Taken together, our new equatorial thermocline Indian Ocean 14 C data points towards extensive, but variable mixing of the Indian Ocean equatorial thermocline with extremely 14 C-depleted abyssal waters. Our study therefore shows that the deep Indian Ocean carbon reservoir, although temporally restricted, expanded to thermocline waters and thus contributed to the drawdown of atmospheric CO 2 at the end of the last glacial period. The dynamic nature of this oceanographic phenomenon suggest that this extended carbon pool is regionally variable. Accordingly, future studies should intensively try to identify regional differences and depth constraints of carbon pool extension especially in the Indian Ocean.

Online methods
Cold-water coral samples. This study analysed scleractinian cold-water corals retrieved during research cruise SO236 to the Maldives Archpielago. In particular, coral samples were collected by a video-guided grab and a box corer in the Vaadhoo Channel (SO236-007, 04°09.07ʹN, 73°29.28ʹE, 443 m water depth) and the Kardiva Channel (SO236-017-TVG, 04°51.26N, 73°28.05ʹE, 453-457 m water depth. Initial radiocarbon datings 32 revealed calibrated ages near the LGM between 22.54 and 21.4 ka. Thus, this sample set provides the unique opportunity to study ventilation ages at thermocline depth of the equatorial Indian Ocean during the LGM. Well-preserved coral skeletons (Desmophyllum pertusum, Enallopsammia rostrata and Madrepora oculata) were cleaned mechanically in order to remove potential containments (e.g., ferro-manganese coatings, borings, epibionts). Samples have been screened for their mineralogy with a PANalytical X'Pert PRO diffractometer, equipped with a copper X-ray tube revealing that all samples remained in their initial aragonitic mineralogy. 230 Th/U ages determinations. Samples were chemically cleaned in a weak acid leach 28 . The 230 Th/U measurements were carried out at the Institute of Environmental Physics at Heidelberg University (IUP, Germany) on a multi-collector inductively coupled plasma mass spectrometer (ThermoFisher, Neptune Plus) 28 . The reference material HU-1 was measured for the reproducibility assessment of the mass-spectrometry measurements 54 . Note, we assume HU-1 to be in secular equilibrium, which contrasts with observations by ref 54 and causes a 1.5‰ difference in the absolute value of δ 234 U. For age determination this difference has no consequence, as we use the half-lives of ref 54 for age determination, hence we presume a different isotopic composition for our batch of HU-1 if compared to the data published by ref 54 . In total, 13 samples were analysed revealing all only minor residual contaminations ( 232 Th < 4 ppb). Nevertheless, an initial 230 Th correction was applied prior to age calculations using a 230 Th/ 232 Th activity ratio for the upper thermocline waters of 8 ± 4 18 . Age determinations and uncertainty assessment were carried out using iterative solution of the decay equations and error propagation using Monte Carlo simulations 26 . The initial 234 U/ 238 U activity ratios of all measured corals are, when transferred into δ 234 U notation (i.e., ‰ deviation from secular equilibrium), within uncertainty in a narrow band of ± 10‰ compared to the value of modern seawater (145.0 ± 1.5‰ 55 ), suggesting a closed system behaviour for the exchange of U between the skeletons and seawater.
Radiocarbon measurements. The extraction of CO 2 from the CWC samples was carried out at the IUP, Heidelberg University, Germany, following the method described in 56 . The final iron-graphite compound was measured on an accelerator mass spectrometer (AMS, MICADAS) at the Curt-Engelhorn-Center Archaeometry (CEZA), Mannheim, Germany 57,58 . Calculation of ∆ 14 C, ∆∆ 14 C and Benthic-Atmosphere (B atm ) ages 10,29,30 is based on IntCal20 33 .
Modelling. The radiocarbon measurements were compared with ∆ 14 C values simulated using an enhanced version of the Hamburg Large Scale Geostrophic ocean general circulation model 59 ; for the enhancements and implementation of ∆ 14 C see refs. 60

Data availability
Data associated to this article can be found in the supplementary online material.